Good day, Chirag Patel4.
Thanks for reaching out to our Microsoft Community Forum. Please accept our warmest regards and sincerest hope that all is well.
We understand that you are having some issues adding users to Microsoft Loop. This is especially the case with old employees of the company.
As explained in the link below, Microsoft Loop is still in Public Preview. As such there are still features that need to be improved on and thus issues like the one you have cannot be resolved right away.
Also, since you're using Microsoft Loop in an enterprise environment, you should contact your company's I.T. department about this issue. As stated in the link above, your I.T. department does have full control of Microsoft Loop.
Regardless of what you I.T. department says or does though, I highly recommend that you provide this experience of yours -- including the 4-5 tickets -- as feedback to our developers directly. They'd love to hear from you about what’s working well, what’s not, and any ideas for improvements including possible new features and functions. Your input will help us improve the user experience for you and all users worldwide.
To send us your feedback, go to the bottom right corner, select Help> Give feedback.

On Android or iOS, tap your profile picture at the top left to find Help and feedback.
Thanks again for choosing Microsoft! We’re here when you need us.
Respectfully,
Glenn Rue
Forum Moderator
Microsoft Community